1. Field of the Invention
The present invention relates to an image processing apparatus and method for correctly extracting digital watermark information from image data having the digital watermark information embedded, and a storage medium for use with this method.
2. Related Background Art
In recent years, with the rapid development and spread of computers and networks, various kinds of information such as character data, image data or voice data are digitized and utilized. The digital information can be retained in its complete state permanently because of no degradation with a secular change over time, while it can be readily copied, and has the significant problem with the protection of copyright.
Therefore, security techniques for the protection of copyright are becoming increasingly important rapidly.
One of the techniques for protecting the copyright is a xe2x80x9cdigital watermarkxe2x80x9d technique. This digital watermark technique is one in which the name of a copyright holder or ID of a purchaser is embedded into the digital image data, voice data or character data in the form insensible to the human eyes to keep track of the unauthorized use due to illegal copy.
The digital watermark technique can be applied not only to the protection of copyright, but also to detecting a falsified position of digital data by embedding beforehand the information into digital data and coordinating the information with the digital data in accordance with the embedding rules.
The digital watermark technique typically relies on a method of embedding various kinds of information (digital watermark information) by applying alteration on the digital data to be insensible to the human eyes, and in the digital data having the digital watermark information embedded, three factors of xe2x80x9cpicture quality compared with the originalxe2x80x9d, xe2x80x9cendurance of digital watermarkxe2x80x9d and xe2x80x9cembeddable amount of informationxe2x80x9d are closely related, and placed in the trade off relation from each other.
The xe2x80x9cpicture qualityxe2x80x9d corresponds to the degree of visual degradation of a digital watermark embedded image with respect to the original image. In the digital watermark technique, since the original image data is manipulated by a minute amount, if the digital watermark information is embedded, the embedded image may necessarily be degraded at the signal level from the original image. However, if this degradation is made insensible to the vision of the human, the embedded image becomes usable without losing the values of the original image. By embedding the digital watermark information, it is preferable that the visual image degradation may not occur.
The xe2x80x9cendurancexe2x80x9d corresponds to the degree at which the digital watermark information can be extracted later without error even when the digital watermark embedded image is subjected to various attacks. The attacks include the geometrical distortion such as enlargement, reduction or dislocation, non-reversible compression such as JPEG compression, partial cutting, noise addition, and density conversion. In a series of operations of outputting the digital watermark information embedded image onto the recording medium such as paper by the printer, inputting it by the scanner, and extracting the digital watermark information, those operations can be considered wholly or partially as the attacks if there is any possibility that the picture quality may change even slightly. It is preferred that the digital watermark information can be extracted correctly even if the image is subjected to these attacks.
The xe2x80x9cembeddable amount of informationxe2x80x9d is defined as the amount of information which can be embedded as the digital watermark information. It is preferred that this embeddable amount of information is as large as possible.
There is the need for an algorithm of embedding the digital watermark information, in which the visual picture quality is not degraded by embedding the digital watermark information, the digital watermark information is endurable against various attacks, and the embeddable amount of information is large.
It is said that there is the trade off relation among those three parameters. For example, if the endurance against the attacks is raised, the image degradation may be aggravated, or the embeddable amount of information may be diminished. On one hand, if the image degradation is to be reduced, the endurance against the attacks may be diminished, or the embeddable amount of information may be reduced. On the other hand, if the embeddable amount of information is to be increased, the image degradation may be worsened, or the endurance against the attacks may be lessened.
Hence, when the digital watermark technique is developed, it is required to examine an optimal method in accordance with the situation where the digital watermark information is embedded.
Conventionally, a patchwork method is well known which is one method for embedding the digital watermark information. This patchwork method is one in which the values in a part of the image are incremented at will, while the values in the other part are decremented at will, so that some appendant information can be embedded while the values for the overall image are retained.
Conventionally, in extracting the digital watermark information from the digital watermark embedded image data by the patchwork method, it is necessary to know the embedding situation of the original image data from the digital watermark embedded image. However, there have been few methods that take into consideration such situation, and no technique has been yet established which extracts the digital watermark information correctly while inferring the situation of the original image data.
Also no technique has been established for correctly extracting the digital watermark information from the image data having the digital watermark embedded by various methods after the image data is subjected to an attack for causing geometrical distortion.
The present invention has been achieved to solve the above-mentioned problems, and its main object is to extract the digital watermark information from the digital watermark embedded image data as correctly as possible.
For example, this invention aims at extracting the digital watermark information as correctly as possible by inferring the state of the image data having no information embedded from the image data having the digital watermark information possibly embedded.
In order to attain the above object, according to a preferred embodiment of the present invention, there is provided an image processing apparatus for extracting the digital watermark information from the image data into which the digital watermark information has been embedded to be less visible to the human eyes, comprising a first extracting unit (similarly corresponding to a utilization information extracting unit 2003) for extracting the first information deriving the digital watermark information by performing an arithmetical operation on the image data using a first pattern arrangement (e.g., corresponding to a pattern arrangement of FIG. 9 in this embodiment), a second extracting unit (similarly corresponding to a second pattern arrangement extracting unit 2005) for extracting the second information indicating the state of the image data having no digital watermark information embedded by performing an arithmetical operation on the image data using a second pattern arrangement (similarly corresponding to a pattern arrangement of FIG. 36), and a test unit (similarly corresponding to a statistical test unit 2006) for testing the reliability of the first information extracted by the first extracting unit based on the second information.
Furthermore, for example, even if any geometrical distortion is applied to the image having the digital watermark information embedded, it is another object of the invention to accurately judging this distortion, and consequently to precisely extract the digital watermark information at a subsequent stage.
In order to attain the above object, according to another preferred embodiment of the invention, there is provided an image processing apparatus for extracting the digital watermark information from the image data into which the digital watermark information including the positional information has been embedded to be less visible to the human eyes, comprising a search unit for performing a process for extracting the positional information from the image data by multiple times at the different extracting start positions, a calculation unit for calculating the reliability indicating the probability of extracting the positional information for each information being extracted as the positional information by the search unit, and a discrimination unit for discriminating the position at which the digital watermark information is embedded in the image data, based on the reliability.
In order to solve the above-object in another way, according to a further embodiment of the invention, there is provided an image processing apparatus for extracting the digital watermark information from a registration signal indicating the geometrical state of the image data and the image data into which the digital watermark information has been possibly embedded to be less visible to the human eyes, comprising a transform unit for transforming the image data to the frequency data, a differentiation unit for differentiating a signal transformed by the transform unit, and a correction unit for performing the geometrical correction of the image data based on a signal differentiated by the differentiation unit and the information concerning the registration signal.
Other objects and features of the present invention will be described below from the ensuing description of the embodiments and the accompanying drawings.